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class="sidebar-link">JS/JSX中的组件名大小写</a></li></ul></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><p>前面我们已经说过Object的检测方式，为啥还要单独把Array拎出来说呢？</p> <p>因为Object.defineProperty无法准确检测到数组的变化，如我们使用push给一个数组添加了元素并不会触发setter和getter方法。</p> <h2 id="如何追踪变化"><a href="#如何追踪变化" class="header-anchor">#</a> 如何追踪变化</h2> <p>我们可以用一个拦截器覆盖Array.prototype。然后每当使用Array原型上的方法操作数组时，执行的都是这个拦截器中的方法，在拦截器中使用原生Array的原型方法去操作数组。</p> <h2 id="拦截器"><a href="#拦截器" class="header-anchor">#</a> 拦截器</h2> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> arrayProto <span class="token operator">=</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype

<span class="token comment">// 此处的分号不可省略，再编写JS代码时并非所有的语句后面都可以省略分号</span>
<span class="token keyword">const</span> arrayMethods <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>arrayProto<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span> 
  <span class="token string">'push'</span><span class="token punctuation">,</span>
  <span class="token string">'pop'</span><span class="token punctuation">,</span>
  <span class="token string">'shift'</span><span class="token punctuation">,</span>
  <span class="token string">'unshift'</span><span class="token punctuation">,</span>
  <span class="token string">'splice'</span><span class="token punctuation">,</span>
  <span class="token string">'sort'</span><span class="token punctuation">,</span>
  <span class="token string">'reverse'</span>
<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">method</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 缓存原始方法</span>
  <span class="token keyword">const</span> original <span class="token operator">=</span> arrayProto<span class="token punctuation">[</span>method<span class="token punctuation">]</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">,</span> method<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">vaule</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token function">mutator</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token function">original</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    enumerable<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    writable<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    configurable<span class="token operator">:</span> <span class="token boolean">true</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><h2 id="使用拦截器覆盖array原型"><a href="#使用拦截器覆盖array原型" class="header-anchor">#</a> 使用拦截器覆盖Array原型</h2> <p>在Observer中处理要被转成响应式的数组类型</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">export</span> <span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value
   
    <span class="token keyword">if</span> <span class="token punctuation">(</span>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token operator">+</span>      value<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> arrayMethods
    <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">walk</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>使用__proto__覆盖原型：</p> <p><img src="/assets/img/3.1dd8209c.png" alt="使用__proto__覆盖原型"></p> <p>可以使用Object.setPrototypeOf来代替__proto__实现同样的效果。</p> <h2 id="将拦截器方法挂载到数组的属性上"><a href="#将拦截器方法挂载到数组的属性上" class="header-anchor">#</a> 将拦截器方法挂载到数组的属性上</h2> <p>上面的代码有个很大问题，并非所有浏览器都支持__proto__属性，Vue中如果浏览器中支持__proto__就直接用__proto__，如果浏览器不支持__proto__就将arrayMethods身上的方法设置到被侦测的数组上</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> arrayMethods <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./array'</span>

<span class="token keyword">const</span> hasProto <span class="token operator">=</span> <span class="token string">'__proto__'</span> <span class="token keyword">in</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>

<span class="token comment">// Object.getOwnPropertyNames()方法返回一个由指定对象的所有自身属性的属性名（包括不可枚举属性但不包括Symbol值作为名称的属性）组成的数组。</span>
<span class="token keyword">const</span> arrayKeys <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyName</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">)</span>

<span class="token keyword">export</span> <span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value

    <span class="token keyword">if</span> <span class="token punctuation">(</span>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">const</span> augment <span class="token operator">=</span> hasProto <span class="token operator">?</span> protoAugment <span class="token operator">:</span> copyAugment
      <span class="token function">augment</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> arrayMethods<span class="token punctuation">,</span> arrayKeys<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">walk</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  <span class="token operator">...</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">protoAugment</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> src<span class="token punctuation">,</span> keys</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  target<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> src
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">copyAugment</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> src<span class="token punctuation">,</span> keys</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> l <span class="token operator">=</span> keys<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i <span class="token operator">&lt;</span> l<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> key <span class="token operator">=</span> keys<span class="token punctuation">[</span>i<span class="token punctuation">]</span>
    <span class="token function">def</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> src<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><img src="/assets/img/4.51925679.png" alt=""></p> <h2 id="如何收集依赖"><a href="#如何收集依赖" class="header-anchor">#</a> 如何收集依赖</h2> <p>通过前面的学习我们知道Object是在defineReactive中的getter中通过Dep收集依赖的，那么数组应该怎么收集依赖呢？</p> <p>实际数组也是在getter中收集依赖的，因为Vue中的数据都定义在data这种属性中，它是一个对象(即便是一个方法，返回的也是一个对象)</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
  <span class="token function">data</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">{</span>
      arr<span class="token operator">:</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">create</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>arr<span class="token punctuation">)</span> <span class="token comment">// 访问数组</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>在我们通过this.arr访问数组时首先会触发arr属性的getter方法，我们可以在这里收集数组依赖</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">defineReactive</span><span class="token punctuation">(</span><span class="token parameter">data<span class="token punctuation">,</span> key<span class="token punctuation">,</span> val</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> dep <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Dep</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> key<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      dep<span class="token punctuation">.</span><span class="token function">depend</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
      <span class="token comment">// 在这里收集Array的依赖</span>
      <span class="token keyword">return</span> val
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Array在getter中收集依赖，在拦截器中触发依赖</p> <h2 id="依赖列表存在哪儿"><a href="#依赖列表存在哪儿" class="header-anchor">#</a> 依赖列表存在哪儿</h2> <p>数组依赖我们应该存放到哪呢？对数组依赖的存放相对麻烦些，因为我们需要在getter中收集依赖，在数组拦截器中触发依赖，然而这两个文件并不在一块，我们需要找个这两个文件都能访问的地方来存放依赖。</p> <p>在Vue源码中把Array的依赖存放在Observer中，因为在getter和数组拦截器中都能访问到Observer的实例</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">export</span> <span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value
<span class="token operator">+</span>   <span class="token keyword">this</span><span class="token punctuation">.</span>dep <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Dep</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token operator">...</span>
  <span class="token punctuation">}</span>
  <span class="token operator">...</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="收集依赖"><a href="#收集依赖" class="header-anchor">#</a> 收集依赖</h2> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">defineReactive</span><span class="token punctuation">(</span><span class="token parameter">data<span class="token punctuation">,</span> key<span class="token punctuation">,</span> val</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> childOb <span class="token operator">=</span> <span class="token function">observe</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span>
  <span class="token keyword">let</span> dep <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Dep</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> key<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>childOb<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        childOb<span class="token punctuation">.</span>dep<span class="token punctuation">.</span><span class="token function">depend</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
      <span class="token punctuation">}</span>
      <span class="token keyword">return</span> val
    <span class="token punctuation">}</span>
      <span class="token operator">...</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
  <span class="token operator">...</span>
<span class="token punctuation">}</span>

<span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">observe</span><span class="token punctuation">(</span><span class="token parameter">value<span class="token punctuation">,</span> asRootData</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">isObject</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span>
  <span class="token punctuation">}</span>

  <span class="token keyword">let</span> ob
  
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">hasOwn</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token string">'__ob__'</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> value<span class="token punctuation">.</span>__ob__ <span class="token keyword">instanceof</span> <span class="token class-name">Observer</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    ob <span class="token operator">=</span> value<span class="token punctuation">.</span>__ob__
  <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
    ob <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Observer</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
  
  <span class="token keyword">return</span> ob
<span class="token punctuation">}</span>
</code></pre></div><p>observe方法尝试为value创建一个Observer实例，如果创建成功，直接返回新创建的Observer实例。如果value已经存在一个Observer实例，则直接返回它，避免了重复侦测value的变化</p> <p>在defineReactive中调用了observe方法，如果传入的参数是一个对象或者数组会得到一个Observer的实例childOb，调用childOb的属性下的depend方法收集依赖。</p> <h2 id="在拦截器中获取observer实例"><a href="#在拦截器中获取observer实例" class="header-anchor">#</a> 在拦截器中获取Observer实例</h2> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">def</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> key<span class="token punctuation">,</span> val<span class="token punctuation">,</span> enumerable</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> key<span class="token punctuation">,</span> <span class="token punctuation">{</span>
        value<span class="token operator">:</span> val<span class="token punctuation">,</span>
        enumerable<span class="token operator">:</span> <span class="token operator">!</span><span class="token operator">!</span>enumerable<span class="token punctuation">,</span>
        writable<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
        configurable<span class="token operator">:</span> <span class="token boolean">true</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">export</span> <span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>
    <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token operator">...</span>
        <span class="token function">def</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token string">'__ob__'</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">)</span>
        <span class="token operator">...</span>
    <span class="token punctuation">}</span>
    <span class="token operator">...</span>
<span class="token punctuation">}</span>

<span class="token comment">// 数组拦截器</span>
<span class="token punctuation">[</span>
  <span class="token string">'push'</span><span class="token punctuation">,</span>
  <span class="token string">'pop'</span><span class="token punctuation">,</span>
  <span class="token string">'shift'</span><span class="token punctuation">,</span>
  <span class="token string">'unshift'</span><span class="token punctuation">,</span>
  <span class="token string">'splice'</span><span class="token punctuation">,</span>
  <span class="token string">'sort'</span><span class="token punctuation">,</span>
  <span class="token string">'reverse'</span>
<span class="token punctuation">]</span>
<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">method</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 缓存原始方法</span>
  <span class="token keyword">const</span> original <span class="token operator">=</span> arrayProto<span class="token punctuation">[</span>method<span class="token punctuation">]</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">,</span> method<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">value</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token function">mutator</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">const</span> ob <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>__ob__ <span class="token comment">// 新增</span>
      <span class="token keyword">return</span> <span class="token function">original</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    enumerable<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    writable<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    configurable<span class="token operator">:</span> <span class="token boolean">true</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>在Observer中为vaule新增了一个不可枚举的属性__ob__值为Observer的实例。如果value是数组的话，当我们调用改变数组方法的时候如<code>value.push(1)</code>就会走到数组拦截器，此时数组拦截器中的this指的就是value，就可以在数组拦截器中通过this.__ob__取得observer的实例</p> <h2 id="向数组的依赖发送通知"><a href="#向数组的依赖发送通知" class="header-anchor">#</a> 向数组的依赖发送通知</h2> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">[</span>
  <span class="token string">'push'</span><span class="token punctuation">,</span>
  <span class="token string">'pop'</span><span class="token punctuation">,</span>
  <span class="token string">'shift'</span><span class="token punctuation">,</span>
  <span class="token string">'unshift'</span><span class="token punctuation">,</span>
  <span class="token string">'splice'</span><span class="token punctuation">,</span>
  <span class="token string">'sort'</span><span class="token punctuation">,</span>
  <span class="token string">'reverse'</span>
<span class="token punctuation">]</span>
<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">method</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 缓存原始方法</span>
  <span class="token keyword">const</span> original <span class="token operator">=</span> arrayProto<span class="token punctuation">[</span>method<span class="token punctuation">]</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">,</span> method<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">value</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token function">mutator</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">const</span> ob <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>__ob__
      ob<span class="token punctuation">.</span>dep<span class="token punctuation">.</span><span class="token function">notify</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// 向依赖发送消息</span>
      <span class="token keyword">return</span> <span class="token function">original</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    enumerable<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    writable<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    configurable<span class="token operator">:</span> <span class="token boolean">true</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>通过前面的操作，我们在数组拦截器中获取到了Observer的实例，就可以调用dep.notify()方法通知依赖发生了变化。</p> <h2 id="侦测数组中元素的变化"><a href="#侦测数组中元素的变化" class="header-anchor">#</a> 侦测数组中元素的变化</h2> <p>回忆下前面代码中的Observer方法中我们只对对象进行了递归侦测，并未对数组中的元素进行处理，下面我们来改进下</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">export</span> <span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value
    <span class="token function">def</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token string">'__ob__'</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">)</span>

    <span class="token comment">// 新增</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">observeArray</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">walk</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  <span class="token function">observeArray</span> <span class="token punctuation">(</span><span class="token parameter">items</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> l <span class="token operator">=</span> items<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i <span class="token operator">&lt;</span> l<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token function">observe</span><span class="token punctuation">(</span>items<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  ……
<span class="token punctuation">}</span>
</code></pre></div><p>新增observeArray方法，用于侦测数组中的每一项</p> <h2 id="侦测新增元素的变化"><a href="#侦测新增元素的变化" class="header-anchor">#</a> 侦测新增元素的变化</h2> <p>上面我们对数组中原有元素的变化进行了侦测，但是对于新增元素如何进行侦测呢？</p> <p>首先我们要获取到哪些是新增的元素，然后对这些新增的元素增加侦测。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">;</span><span class="token punctuation">[</span>
  <span class="token string">'push'</span><span class="token punctuation">,</span>
  <span class="token string">'pop'</span><span class="token punctuation">,</span>
  <span class="token string">'shift'</span><span class="token punctuation">,</span>
  <span class="token string">'unshift'</span><span class="token punctuation">,</span>
  <span class="token string">'splice'</span><span class="token punctuation">,</span>
  <span class="token string">'sort'</span><span class="token punctuation">,</span>
  <span class="token string">'reverse'</span>
<span class="token punctuation">]</span>
<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">method</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 缓存原始方法</span>
  <span class="token keyword">const</span> original <span class="token operator">=</span> arrayProto<span class="token punctuation">[</span>method<span class="token punctuation">]</span>
  <span class="token function">def</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">,</span> method<span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token function">mutator</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token function">original</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span>
    <span class="token keyword">const</span> ob <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>__ob__
    <span class="token keyword">let</span> inserted
    <span class="token keyword">switch</span> <span class="token punctuation">(</span>method<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">case</span> <span class="token string">'push'</span><span class="token operator">:</span>
      <span class="token keyword">case</span> <span class="token string">'unshift'</span><span class="token operator">:</span>
        inserted <span class="token operator">=</span> args
        <span class="token keyword">break</span>
      <span class="token keyword">case</span> <span class="token string">'splice'</span><span class="token operator">:</span>
        inserted <span class="token operator">=</span> args<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
        <span class="token keyword">break</span>
    <span class="token punctuation">}</span>
    ob<span class="token punctuation">.</span>dep<span class="token punctuation">.</span><span class="token function">notify</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token keyword">return</span> result
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>在数组拦截器中我们通过switch...case对增加数组元素的方法进行了处理，将新增的元素存放到了inserted变量中，完成了获取新增元素的任务，下面来使用Observer侦测新增元素。</p> <p>回想下，前面我们对数组元素进行侦测使用的是observeArray方法，对于新增方法是否也可以使用这个方法呢？</p> <p>答案当然是可以的，我们在数组拦截器中可以通过this.__ob__获得Observer的实例，因此可以访问到observeArray方法，如果有新增元素的话直接调用observeArray方法对新增元素进行侦测即可。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">;</span><span class="token punctuation">[</span>
  <span class="token string">'push'</span><span class="token punctuation">,</span>
  <span class="token string">'pop'</span><span class="token punctuation">,</span>
  <span class="token string">'shift'</span><span class="token punctuation">,</span>
  <span class="token string">'unshift'</span><span class="token punctuation">,</span>
  <span class="token string">'splice'</span><span class="token punctuation">,</span>
  <span class="token string">'sort'</span><span class="token punctuation">,</span>
  <span class="token string">'reverse'</span>
<span class="token punctuation">]</span>
<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">method</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 缓存原始方法</span>
  <span class="token keyword">const</span> original <span class="token operator">=</span> arrayProto<span class="token punctuation">[</span>method<span class="token punctuation">]</span>
  <span class="token function">def</span><span class="token punctuation">(</span>arrayMethods<span class="token punctuation">,</span> method<span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token function">mutator</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token function">original</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span>
    <span class="token keyword">const</span> ob <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>__ob__
    <span class="token keyword">let</span> inserted
    <span class="token keyword">switch</span> <span class="token punctuation">(</span>method<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">case</span> <span class="token string">'push'</span><span class="token operator">:</span>
      <span class="token keyword">case</span> <span class="token string">'unshift'</span><span class="token operator">:</span>
        inserted <span class="token operator">=</span> args
        <span class="token keyword">break</span>
      <span class="token keyword">case</span> <span class="token string">'splice'</span><span class="token operator">:</span>
        inserted <span class="token operator">=</span> args<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
        <span class="token keyword">break</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>inserted<span class="token punctuation">)</span> ob<span class="token punctuation">.</span><span class="token function">observeArray</span><span class="token punctuation">(</span>inserted<span class="token punctuation">)</span> <span class="token comment">// 新增</span>
    ob<span class="token punctuation">.</span>dep<span class="token punctuation">.</span><span class="token function">notify</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token keyword">return</span> result
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><h2 id="关于array的问题"><a href="#关于array的问题" class="header-anchor">#</a> 关于Array的问题</h2> <p>Vue对数组的变化侦测是通过拦截原型方法的方式实现的，因此一些非原型方法的操作就拦截不到。如：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">this</span><span class="token punctuation">.</span>list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">1</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>list<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</span>
</code></pre></div><p>上面这样修改数组元素值和清空数组的操作Vue就侦测不到，也就没办法响应。因为在ES6之前无法做到模拟数组的原生行为。ES6提供了元编程的能力有能力拦截，所以Vue3使用ES6提供的Proxy来实现这个功能。</p> <h2 id="总结"><a href="#总结" class="header-anchor">#</a> 总结</h2> <p>Array追踪变化的方式和Object不一样。因为它是通过方法来改变内容的，所以我们通过创建拦截器去覆盖数组原型的方式来追踪变化。</p> <p>为了不污染全局Array.prototype，我们在Observer中只针对那些需要侦测变化的数组使用 <strong>proto</strong> 来覆盖原型方法，但 <strong>proto</strong> 在ES6之前并不是标准属性，不是所有浏览器都支持它。因此，针对不支持 <strong>proto</strong> 属性的浏览器，我们直接循环拦截器，把拦截器中的方法直接设置到数组身上来拦截Array.prototype上的原生方法。</p> <p>Array收集依赖的方式和Object一样，都是在getter中收集。但是由于使用依赖的位置不同，数组要在拦截器中向依赖发消息，所以依赖不能像Object那样保存在defineReactive中，而是把依赖保存在了Observer实例上。</p> <p>在Observer中，我们对每个侦测了变化的数据都标上印记 <strong>ob</strong>，并把this（Observer实例）保存在 <strong>ob</strong> 上。这主要有两个作用，一方面是为了标记数据是否被侦测了变化（保证同一个数据只被侦测一次），另一方面可以很方便地通过数据取到 <strong>ob</strong>，从而拿到Observer实例上保存的依赖。当拦截到数组发生变化时，向依赖发送通知。</p> <p>除了侦测数组自身的变化外，数组中元素发生的变化也要侦测。我们在Observer中判断如果当前被侦测的数据是数组，则调用observeArray方法将数组中的每一个元素都转换成响应式的并侦测变化。</p> <p>除了侦测已有数据外，当用户使用push等方法向数组中新增数据时，新增的数据也要进行变化侦测。我们使用当前操作数组的方法来进行判断，如果是push、unshift和splice方法，则从参数中将新增数据提取出来，然后使用observeArray对新增数据进行变化侦测。</p> <p>由于在ES6之前，JavaScript并没有提供元编程的能力，所以对于数组类型的数据，一些语法无法追踪到变化，只能拦截原型上的方法，而无法拦截数组特有的语法，例如使用length清空数组的操作就无法拦截。</p></div> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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